The ever-decreasing size of microelectronic devices and the rapid development of microelectromechanical systems (MEMS) have created a great need for high energy density micropower supplies, for example, a power supply for microelectronic devices. Typically, conventional battery technology is used in these applications. However, current battery technology has a very low energy density, on the order of from 0.035 to 0.350 kWe-hr/kg. An alternative to batteries is to combine a small fuel cell with a micro-hydrocarbon fuel processor. However, thus far, it has not been possible to construct a very small, thermally efficient fuel reformer. An additional problem is that many fuel cells require hydrogen gas having very low levels of carbon monoxide (CO) contamination. Therefore, it is also desirable for a microreformer to produce hydrogen that contains very little CO. Another problem is that instability in microcombustor operation can lead to partial vaporization of the hydrocarbon fuel, if it is liquid, and to less than desired conversion of the hydrocarbons to a hydrogen rich product stream due to the intermittent lack of energy for the endothermic reactions.
Prior attempts to lower CO in a reformate mixture have included: a two stage methanation process conducted at two temperatures over a 2% Rh/alumina catalyst (Van Keulen, U.S. Pat. No. 6,207,307); passage of the reformate through a palladium membrane followed by methanation of residual CO over a catalyst such as Ru, Rh, Pd, Ir, Pt, Ni and Re (Soma et al., U.S. Pat. No. 5,612,012); passage of the reformate through a hydrogen selective membrane followed by methanation of residual CO (Edlund, U.S. Pat. No. 5,861,137); and heating a gas in the presence of a water-gas shift catalyst to reduce the CO content to about 3000 parts per million (ppm), removing water, followed by reaction over Ru or Rh on alumina at below 250 C (Baker et al., U.S. Pat. No. 3,615,164).
Bohm et al. in U.S. Pat. No. 5,904,913 stated that methanol can be reformed at 220 to 280° C. over a Cu/ZnO on alumina catalyst. Bohm et al. reported that they had found that in their apparatus, for a methanol conversion above 98%, with a maximum reaction tube length of 160 cm, a reaction temperature of at least 260° C. should be selected. Lower temperatures would require longer reaction tube lengths. In their apparatus, for a catalyst loading of 1.3 kg, a productivity of 8 Nm3H2/h was achieved, which required a minimum temperature of 280° C. for 100% methanol conversion. To lower CO, output from the reforming reaction tubes can be passed to a CO converter to methanate the CO over a titania/alumina/Ru/RuOx catalyst with a Ru/RuOx fraction of between 2 to 4% at a maximum temperature of about 200° C.
The prior art processes for reforming hydrocarbons to produce hydrogen suitable for a fuel cell typically require multiple step operations in large and complex apparatus. Thus, there remains a need for microcombustors and fuel reformers which have a very small size, steady performance, and operate at low temperature with low CO output while maintaining high efficiency levels.
The development of better steam reforming catalysts has long been an area of intense interest. An example of some recent research appears in published patent application EP 1 061 011 A1. In this publication, Wieland et al. report a supported PdZn/ZnO catalyst for methanol steam reforming. A catalyst (Example A) was made by wash coating gamma-alumina onto a ceramic honeycomb, impregnating the gamma-alumina with an aqueous solution containing Pd(NO3)2 and Zn(NO3)2, followed by drying, calcining at 500° C. and reducing at 400° C.
Despite years of intense research, there remains a need for steam reforming catalysts and steam reforming methods with improved performance in terms of H2 productivity, low temperature activity, and low CO selectivity.